Many standard light sources such as lights in buildings are essentially unpolarized. Images that are hidden when viewed with unpolarized light but can be revealed to observers using simple means of detection such as polarizers are useful tools for covert security marking, security printing and tagging applications. In addition, security markings that are visible to the naked eye but their appearance vary with changing the polarization states of the illumination are important too.
In U.S. Pat. No. 6,740,472, Karasev described a security latent (hidden) image system of which the hidden information can be revealed by using a circular polarizing filter. However, due to the nature of the invention, this security marking system is limited only to markings on essentially specular reflective substrate, such as a metallic coating, which is a sever limitation on the possible applications. In addition, the method of image formation requires a complicated process of UV radiation through a specially manufactured mask.
US patents (Schadt, et al.) U.S. Pat. Nos. 6,144,428 and 7,292,292 B2 describe a complicated system of three different layers comprising a cholesteric liquid crystal polymer layer, a linear-photo-polymerization (LLP) alignment layer, and a (nematic) liquid crystal polymer structured (patterned) retarder layer. Hidden images are written in the LLP layer by exposing the relevant parts to different orientations of linearly polarized UV radiation via masks. Such images are generally not visible under unpolarized light, but become visible when viewed through linear polarizers. There is no limitation on choice of substrates.
Both of the above-mentioned technologies require the complicated, expensive and slow processes of photo radiation through specially prepared masks. In the case of U.S. Pat. No. 6,740,472, the substrates are limited to metallic or metallic-like; in the case of Schadt, multiple layers of coatings, and multiple UV exposure steps are required. Since the above security systems require masks they are useful for manufacturing of identical security marks but are not practical if the marks are required to be changed frequently. In particular, they are not practical if each mark has to be different from all other marks as in serial numbers marks.
The above shortcomings can be overcome by a method disclosed by Faris in U.S. Pat. No. 6,133,980. In this patent, an optical element comprises a surface retarder on a liquid crystal film, such as cholesteric liquid crystal polymer (CLCP) film, is described. Also disclosed, but not claimed, are three methods of making such optical element. Compare to Karasev and Schadt, this prior art offers a much simpler way of creating hidden images in a single layer of polymer. However, U.S. Pat. No. 6,133,980 lacks sufficient information on the fabrication process for making such optical elements and it is not very useful for practical applications.
The current invention teaches fabrication processes that are reproducible for fabricating the above-mentioned optical element of good quality and at low cost. Specifically, the current invention teaches the fabrication of optical security devices using our improved fabrication process. The hidden images are patterned directly on the top surface of the CLCP, and are embedded in the CLCP film by locally modifying the alignment of the cholesteric molecules in desirable directions to create optical retarders on the surface. Creating multiple retarders with axes at different directions allows the creation of multiple hidden images in a single layer. The hidden images are detectable by viewing the optical security device through either linear or circular polarizers. Alternatively, polarized light sources can be used (polarizer is located near the light source) and, therefore, can be viewed directly without having to look through a polarizer. Our invention thus allow to fabricate covert marks in a single layer and doing so in a way that is compatible with roll-to-roll process that are essential to reduce the fabrication cost of the security devices.